The formation of microelectronic devices on semiconductor substrates requires, for certain applications, the formation of mechanically isolated semiconductor microstructures, such as beam's and diaphragms. Such microstructures are important components of a variety of microsensors, including pressure transducers, strain sensors, and vapor and humidity sensors. Microstructures are currently being explored for applications beyond typical microelectronics devices, for example in the formation of micromachines which include highly miniturized parts such as gears and levers.
The formation of micromechanical devices and other microstructures in semiconductors often utilizes surface machining. Such machining operations begin with a stiff substrate, for example, a silicon wafer, which is covered by a sacrificial layer such as silicon dioxide. The sacrificial layer is typically quite thin, in the range of up to two microns or so, and in some cases even thinner. The sacrificial film layer is covered with the desired thin film mechanical construction material, for example, polycrystalline silicon (polysilicon). Thicknesses for the mechanical construction film are typically in the range of a few microns or less. The construction film is patterned into the desired mechanical structure, for example a doubly clamped beam with support pads which are much larger than the width of the beam. The sacrificial layer is then removed from the body of the beam by an etching step which does not attack the substrate or the construction film. The end result should be a freestanding beam, clamped at both ends, with an air gap between the beam and the substrate. The air gap is quite small, the thickness of the sacrificial layer. However, experience has shown that successful formation of such freestanding micromechanical beams only occurs reliably for structures which are relatively quite stiff and with relatively rough substrate and beam surfaces. Mechanically weak structures, which are the rule rather than the exception in micromechanical structures, normally deflect and attach themselves permanently to the substrate surface at all points at which the construction film and the substrate come into contact.
The problem of bonding between the construction film and the substrate is related to three physical phenomena. These are (1) surface bonding due to an etch residue compound left on hydrogen fluoride etched structures, (2) surface tension forces during the drying phase of the wet chemistry procedures used to create the isolated mechanical structure, and (3) electrostatically induced deflections due to built-up potentials between the free beam or diaphragm and the substrate.
Hydrofluoric acid, HF, is the only effective etchant presently used for silicon dioxide on silicon processing technology. It has been proposed that the HF etching process leaves a residue which is either a hydrocarbon or a fluorocarbon and which cannot be removed by extensive rinses with deionized water. See G. Gould, et al., "An In Situ Study of Aqueous HF Treatment of Silicon by Contact Angle Measurement and Ellipsometry," J. Electrochem. Soc.: Solid State Science and Technology, Vol. 135, No. 6, June 1988 pp. 1535-1539. Although the compound has not been identified, the experimental observations are consistent with this proposal. For example, if a doubly supported beam is etched free with a HF etching, and the resulting structure is dilution rinsed for up to two days without exposure to room ambient, drying of the structures will cause permanent attachment of the beam to the substrate.
It is noted that a wafer which is completely covered by a film of water cannot experience surface tension forces, and it will therefore exhibit undeflected beams during microscopic examination in the presence of water. If, however, the beams are dried and attach themselves to the substrate via chemical bonding, reimmersion of the wafer into water should not produce straight beams if the bonding compound is not water soluble. This result has, in fact, been observed. It has also been observed that the bonding compound is soluble in HF. Thus, bonded beams can be freed in water by first etching in HF and then following with a dilution rinse. The wafer will again exhibit straight beams as long as it is covered with water.
It is not surprising that two silicon surfaces in contact with one another bond chemically. The silicon chemical bonding effect is the basis for the concept of fusion bonding. In this technology clean, flat semiconductor wafers are first treated in acid baths to produce hydrophyllic surfaces after a subsequent wafer rinse. The wafers are then dried and wafer to wafer bonding occurs when the two polished surfaces are brought into contact. Room temperature adhesion is strong, and heating of the attached wafers to 1000.degree. C. in nitrogen produces excellent stable bonding. The proposed chemical explanation for the bonding involves a hydroxide bond which decomposes thermally, which is somewhat in conflict with the proposed HF residue theory of bonding. In any event, the same type of bonding which is exploited to attach wafers to one another in fusion bonding must also occur for identical facing surfaces in micromechanical structures where such bonding is definitely not desired. It has been possible to treat the wafers chemically to avoid the surface bonding problem in dry ambients, which require that bondable surfaces cannot touch each other until the wafer is dry. This result has not been heretofore achieved in wet ambients.